It was discovered in the 1830s that exposing an electrolytic cell to light increased the amount of current it generated. But the first commercial photovoltaic (PV) products arrived only in the 1950s, when silicon PV devices produced electricity with an efficiency of 4.5 percent. By 1960, the efficiencies of research devices reached 14 percent and commercial products 10 percent.
The simplest PV cell starts out as a wafer of crystal silicon, in which each atom forms covalent bonds with four adjacent atoms in a highly ordered lattice. A potential is obtained by doping the silicon with boron and phosphorus. When boron, with three valence electrons, bonds at a site normally occupied by a silicon atom, with four, one bond must accept an electron from the crystal lattice. Left behind is a positively charged mobile carrier (or hole) that with its fellows forms p-type material. Phosphorus, with five valence electrons, donates an electron to the lattice to create n-type material. The ionized impurity is fixed in the silicon crystal lattice, which contains a balancing number of mobile electric charge-carriers of opposite charge. Boron normally is contained in the wafers used for PV cells in minute, part-per-million concentrations. As for the phosphorous, a high-temperature process diffuses it into the wafer surface in slightly greater concentration than the boron background. Other dopant atoms which produce a p-type material in silicon are the elements of group III of the periodic table, such as aluminum, gallium and indium. Other dopant atoms which produce a n-type material in silicon are the elements of group IV of the periodic table, such as arsenic and antimony.
The junction between n-type and p-type material is referred to as a p-n junction. This junction drives all the mobile charge from the region, leaving a built-in potential sustained by the fixed ionized acceptors and donors. When the silicon absorbs a photon, the event frees an electron to become a mobile carrier and simultaneously creates a hole. The p-n junction, with its built-in electric field, can separate the electron and hole, and the current thus generated can flow when the device is connected to an external circuit. Device efficiency is defined as the ratio of the electric power produced to the power of the incident light, or photons.
Today, the efficiency of cells made in polycrystalline wafers is nearly as good as for single-crystal types. Even greater reductions in materials cost can be achieved if the promise of thin-film technologies is realized. Standard silicon wafers are 200–500 μm thick, but with recently improved optical designs only 10 μm of silicon could be sufficient to capture all the available light. Other materials, such as amorphous silicon or copper indium diselenide (CIS), have an inherently higher absorption coefficient that can shrink the required thickness to 1 μm or less, but at present are only a small portion of the market for solar cells.
The bulk of the PV business builds on the traditional silicon solar cell. Silicon ingots are formed by single-crystal growth on a seed pulled from the silicon melt (a process accounting for 40 percent of sales at present) or by casting large polycrystalline blocks (44 percent of sales). Wafers are sliced from the ingots and turned into full-wafer diodes by high-temperature diffusion processes that minutely modify the surface-layer chemical composition to form a p-n junction. The PV devices are typically completed with electrical contacts and anti-reflection coatings.
Solar cells comprising III–V semiconductors such as gallium arsenide, gallium aluminum arsenide and gallium indium arsenide phosphide (GaAs, GaAlAs, and GaInAsP) have been combined in multiple junctions to reach high efficiencies.
Bandgap Energy (Eg)
Semiconductor materials used for solar cells have a characteristic energy bandgap Eg. This bandgap is characteristic of the material. Bandgap is measured in units of electron volts (eV). Some typical bandgaps are 0.67 eV for germanium, 1.1 eV for silicon, and 1.4 eV for gallium-arsenide. Lower bandgap materials absorb a wider range of light. The cut-off wavelength is the longest wavelength of light that can be usefully absorbed by a semiconductor. This cutoff wavelength is related to the bandgap Eg of the semiconductor by the relation
      λ    c    =      hc    Eg  where h is Planck's constant, and c is the speed of light.
Thus, the lower the bandgap of the semiconductor, the more light can (in principle) be usefully absorbed, and the higher the photocurrent of a solar cell made from that semiconductor.
The voltage produced by a solar cell increases as the bandgap increases. Since the power produced is proportional to the product of the voltage and the photocurrent, there exists an optimum bandgap for a single junction solar cell. For a solar cell operation at an operating temperature of about 25 degrees Celsius, this optimum bandgap is near 1.5 eV.
Multiple Junction Solar Cells
The highest efficiency solar cells manufactured today use the technique of multiple junctions with “subcells” of different bandgaps of semiconductor material. Each subcell is a layer of a semiconductor with a p-n collection junction. The layers are placed on top of each other. This technique splits the solar spectrum into several bands, each one absorbed by a semiconductor layer which is chosen to most efficiently use the spectral range. The topmost layer is the highest bandgap, and absorbs the shortest wavelength light. The next layer down is the next highest bandgap, and absorbs slightly longer wavelength light, and so downward until the lowest layer is the lowest bandgap material, absorbing the longest wavelength light. For example, commercial “triple junction” solar cells use a top layer of gallium indium phosphide semiconductor, a second layer of gallium arsenide semiconductor, and a bottom layer of germanium semiconductor. The materials are chosen to have the same crystalline lattice constant so that the layers can be deposited by the technique of epitaxy, well known in the art to produce layers with a low level of crystalline flaws and defects. It is possible to make layers from semiconductors which are not precisely matched in lattice constant, but in general this technique produces a lower quality material, and only lattice-matched (or nearly lattice-matched) materials are currently used in state-of-the-art high efficiency solar cells.
Solar cells with multiple junctions connected in series with one another are sometimes referred to in the literature as “tandem” solar cells, or “cascade” solar cells. Cells with two junctions are referred to as “dual” junction solar cells, and cells with three junctions as “triple” junction solar cells.
The subcells are typically connected to one another by a semiconductor tunnel junction which serves to connect the subcells electrically, in series with one another. (When the number of junctions is counted, the tunnel junction layers are conventionally omitted. For example, a “dual” junction solar cell has a top p-n junction (or subcell), a bottom p-n junction (or subcell), plus a tunnel junction connecting the two subcells.
Layers connected electrically in series are constrained to have the same current flowing through each layer. For a multiple junction solar cell to work efficiently, the bandgaps of the semiconductor layers must be carefully chosen so that the photocurrent produced by each layer is the same. Either the bandgap of the material or the thickness of the layer (or both) is adjusted to make this match in current.
The epitaxy process grows semiconductor layers on a substrate wafer, usually a single-crystal semiconductor, which provides mechanical support. The highest efficiency solar cells commercially manufactured today use germanium substrate wafers. This material is lattice matched to the wider bandgap semiconductors gallium arsenide and gallium-indium phosphide, producing a three-junction solar cell.
High Temperature Operation
One difficulty of solar cells for some applications is that they do not operate efficiently at elevated temperatures. For example, a typical gallium arsenide solar cell loses approximately 0.2% of its initial power for each rise of one degree Celsius (C) in operating temperature. At an operating temperature of 450 C, the power output is reduced to only 20% of the output at 25 C. A typical silicon solar cell has even worse temperature performance, losing about 0.45% of its initial power for each degree of increase in operating temperature. Above about 250 C, silicon solar cells produce essentially zero power.
The amount of power decrease with temperature depends on the bandgap of the semiconductor material which the solar cell is made from, and the higher the bandgap, the lower the power lost at elevated temperature.
The photocurrent has very little dependence on temperature (in fact, a slight increase with rising temperature). The decrease in performance is primarily due to a decrease in voltage at the operating point.
U.S. Pat. No. 4,582,952 discloses Gallium Arsenide Phosphide Top Solar Cell. A tandem solar cell includes a gallium arsenide phosphide top solar cell and silicon bottom solar cell. The gallium arsenide phosphide solar cell is fabricated on a transparent gallium phosphide substrate and either placed in series with the silicon solar cell for a two terminal device or wired separately for a four terminal device. The top solar cell should have an energy gap between 1.77 and 2.09 eV for optimum energy conversion efficiency. A compositionally graded transition layer between the gallium phosphide substrate and the active semiconductor layers reduces dislocations in the active region. A gallium phosphide cap layer over the gallium arsenide phosphide solar cell reduces surface recombination losses. A difficulty with this design is that Gallium Arsenide Phosphide of energy gap 1.77 to 2.09 eV is not lattice-matched to the silicon crystal, which makes growth of high quality material difficult.
U.S. Pat. No. 4,295,002 discloses heterojunction V-groove multijunction solar cell. In this solar cell, the semiconductor body 13 is either undoped or lightly doped with conductivity type determining impurities. An etching mask, such as an oxide layer 22, which initially provided for the dimension control of the etching of the body 13 from the starting silicon wafer, covers the top of the body 13. The sides 15 and 17 of body 13 are undercut. Illustratively, a region of n-conductivity type gallium phosphide 14 is provided along side 15. Similarly, a region of p-conductivity type gallium phosphide 16 is provided along side 17. The GaP regions 14 and 16 may be formed by angled molecular beam epitaxial deposition. A discontinuous metal layer, e.g., of aluminum, is then provided by vacuum deposition. The metal layer is in three parts 18, 20, 24 due to the undercut of oxide layer 22. It provides electrical contact between the n-type and p-type gallium phosphide regions on adjacent silicon bodies. In addition, the discontinuous metal layer provides an optically reflecting cover for the sunlight being converted in the body 13.
Terminology
A semiconductor junction commonly consists of two layers of semiconductor of opposite conductivity type. The two semiconductor conductivity types are p type, in which the majority conductor is a hole (of positive charge), and n type, in which the majority conductivity type is an electron (of negative charge). P-type and N-type are thus opposite conductivity types. The conductivity type is determined by a small amount of an element that produces free electron or holes in the semiconductor crystalline lattice; this is known as the “dopant”, and the semiconductor with the dopant in the lattice is referred to as “doped”. The junction between the semiconductors is a p-n junction. In a solar cell, a p-n junction collects and rectifies photocurrent produced by sunlight.
A p-n junction can be either a homojunction, in which the two layers are both made from the same semiconductor material (for example, both made from silicon), or a heterojunction, in which the two layers are made from different semiconductor materials (for example, a p-type silicon layer and a n-type gallium-phosphide layer).
Another type of junction is called a tunnel junction. A tunnel junction is a p-n junction with a high amount of doping. The high amount of doping allows current flow in the direction opposite to the normal p-n junction current flow.
A multiple junction solar cell consists of a number of individual subcells, each with a p-n junction, connected electrically in series with one another.
In a solar cell with multiple junctions, in order for voltages to add, the different junction layers need to have the same polarity; that is, if the top layer has p on the top (sun-facing) side and n on the bottom, the lower layer will also have p on the top and n on the bottom. Both p-on-n and also n-on-p solar cells can be fabricated. In order to connect each layer with the next layer, a tunnel junction is used. The tunnel junctions will have the opposite polarity, that is, if the top cell has a p-on-n structure, the tunnel junction connecting it to the next cell will have a n-on-p structure. This technique is well known in the art, and is used for highly efficient solar cells in commercial production.
An alternate technique for connecting layers of a multi-junction cell is to use ohmic metal interconnections between the layers. The metal is suitably gridded or spotted (patterned) to allow light to penetrate to the front surface of the solar cell. This is well known in the art.
Solar cells also have ohmic contacts to allow electrical connection to the cell, consisting of metal. The metal on the top (sun facing) surface is normally a partially transparent or gridded contact, in order to allow sunlight to reach the cell. The contact on the back side can be either gridded or continuous. Ohmic contacts may consist of several layers of metal. Again, ohmic contacts to solar cells are well known in the art, and are used in all conventional solar cells.
It is known to dispose a “window layer” on the sun-facing surface of a solar cell. The window layer is a layer of wide-bandgap semiconductor material of the same doping type as the material beneath it, so that a p-n junction is not formed. The purpose of the window layer is to reduce the loss of photo-generated carriers at the surface of the active layer of the semiconductor. For example, a typical gallium-arsenide solar cell uses an aluminum arsenide or aluminum-gallium-arsenide window layer on the front surface. A window layer is not usually considered an active semiconductor layer, since the primary purpose of the window layer is not to generate carriers.